There are three types of Ni-based superalloy, polycrystal, unidirectional solidification, and single crystal depending on the casting method. Regarding the strength, the Ni-based single crystal superalloy has the highest strength, then the Ni-based unidirectionally solidified superalloy and the Ni-based polycrystalline superalloy in this order.
In addition, the Ni-based superalloy part using the above-described superalloys is used, for example, as a turbine rotor blade or a turbine vane of a jet engine, a gas turbine, or the like. For example, in a jet engine, the turbine inlet gas temperature is a high temperature of 1000° C. to 1700° C., which is equivalent to the melting point of nickel of 1453° C. and the melting point of iron of 1535° C. In general, the higher the turbine inlet gas temperature is, the higher the energy efficiency is, and the rise in fuel price is offset by the improvement of engine efficiency.
On the other hand, since the turbine inlet gas temperature is often higher than the melting point temperature of nickel, a turbine rotor blade and a turbine vane are treated as expendables even in a case where thermal barrier coating is applied. For example, when a jet plane takes off, if the thermal barrier coating of the jet engine parts is peeled off by a cloud of dust or the like, the damage of a turbine rotor blade and a turbine vane progresses during a flight. The damaged turbine rotor blades and turbine vanes are exchanged during the periodic inspection. The replaced turbine rotor blades and turbine vanes are repaired (there may be a case where a regeneration treatment as described in Patent Literatures 1, 2, 3, and 4 described later is included) if the damage is minor, and are discarded if the damage is severe. Repeated repairs will be limited to several times at most, therefore, currently all of the turbine rotor blades and turbine vanes are eventually discarded.
Therefore, in Patent Literature 1, a method for regenerating a turbine rotor blade and a turbine vane, which are made of a cast polycrystalline Ni-based superalloy and have damaged by creep, has been proposed (hereinafter, the process to be performed for the purpose of repairing a part is referred to as regeneration, and the process to be performed for the purpose of reusing an unrepairable part as the part again is referred to as recycling). Further, in Patent Literature 2, it has been proposed that in a case where the strength is deteriorated due to the use at high temperature, by performing a complete solution heat treatment and an aging heat treatment on the Ni-based superalloy material, the remaining life of the single crystal material is extended.
The regeneration method in Patent Literatures 1 and 2 is a method for regenerating a metal structure on a Ni-based superalloy material by a regeneration heat treatment. The regeneration heat treatment is a method of restoring the deteriorated microstructure of a metal only by a heat treatment, the deteriorated structure is once extinguished by a solution treatment, and then a sound fine precipitated structure is regenerated again by an aging treatment. For this reason, in a case where the degree of the damage to a turbine rotor blade and a turbine vane is large, the turbine rotor blade and turbine vane cannot be coped with by the regeneration heat treatment, and have to be discarded.
Furthermore, in Patent Literature 3, in a case where the strength of a Ni-based superalloy part is deteriorated due to the use at high temperature, when a rejuvenation treatment, a regeneration treatment, or a repairing is performed, a heat-resistant protective layer is once removed, then a corrosion layer, an oxide layer, a corrosion product, and an oxidation product are also removed, and further cracks of the heat-resistant protective layer are repaired. After that, it has been proposed to recoat the heat-resistant protective layer.
In addition, in a Ni-based superalloy part, with respect to abrasion and cracks of the alloy substrate, repair of the abrasion and cracks is performed while maintaining the single crystal by the method shown, for example, in Patent Literature 4. However, eventually, when the product life has run out and the product becomes a used scrapped material, the product cannot be regenerated by the regeneration treatment proposed in Patent Literatures 1, 2, 3, and 4, and only some expensive elements are refined from a part material at a high cost by the method shown, for example, in Patent Literature 5.
In general, it is known that a Ni-based superalloy part is highly susceptible to contamination by impurity elements (see, for example, Non Patent Literature 1). In fact, in the conventional cast polycrystalline turbine rotor blades, turbine vanes, and the like, creep characteristics, thermal fatigue characteristics, and environmental resistance characteristics of a superalloy recycled by remelting cannot be guaranteed. For this reason, recycling while maintaining almost the composition of the superalloy by remelting of the Ni-based superalloy part once used has not been performed at all by airline companies and power generation companies that own jet engines and gas turbines that are products.
On the other hand, there may be a case where a NI-based superalloy part contains an expensive rare metal such as hafnium, and rhenium. Rare metals have price problems, and the resource-rich countries are unevenly distributed, therefore, it may be difficult to stably secure the supplier. For this reason, when trying to produce a new product, under the influence of price fluctuations and a supply risk of rare metals, a situation where it is difficult to provide a product at the delivery time and the price, which have been contracted with a customer, frequently occurs (see, for example, Rhenium export suspension measures in Kazakhstan in 2007, in Non Patent Literature 2), and the price stabilization and the stable supply have been strongly demanded.
Accordingly, the present inventors have proposed a production process of a recycled product while maintaining almost the composition of the superalloy by remelting the Ni-based superalloy part once used in Patent Literature 6. However, at the present stage, a recycled product has not been used yet by airline companies and power generation companies.